Light-induced changes in intracellular free calcium concentration, free (Ca ions) in, will be measured in vertebrate rod outer segments. This will be done by using the newly developed technique of metallochromic indicator dyes, of which Arsenazo III is a useful example. Also using this technique, the mechanism by which invertebrate photoreceptors maintain free (Ca ions)in at a low value will be explored. Preliminary results have indicated that injection of Cl ions into vertebrate rod outer segments causes depolarization of the plasma membrane which is not due to a direct electrochemical effect of Cl ions upon the membrane. The mechanism for this depolarization will be explored with further injections performed in media of various ionic compositions. A number of mutants of Drosophila are available which have abnormal photoreceptor potentials. These mutants will be screened for defective Ca ions metabolism by using the metallochromic indicator dye, Arsenazo III. Mutants with defective Ca ions metabolism will be subjected to further analysis. Four newly discovered visual mutants of the mouse will be subjected to electrophysiological analysis to ascertain whether or not the photoreceptors are defective. A newly developed preparation that allows recordings to be made intracellularly from single squid photoreceptors will be exploited in order to complete an electrophysiological analysis of these cells. Tracer experiments will then be done to study the unidirectional Ca ions fluxes across the plasma membrane.